restful: apikey and oauth2 authentication support
- Adds support for API authentication via both API keys and OAuth 2.0.
- Adds pending deprecation messages to API authentication, which will be replaced by OAuth 2.0 personal tokens authentication.
- Fixes an issue where API response headers would include a set-cookie header.
- Aligns unauthenticated response status code to 401 for API key and OAuth authentication.
- Downgrades empty session log message from error level to debug level.
Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>